Hypoxia (a reduction in the normal level of oxygen tension) is a common feature of both experimental and human solid tumors. It results from an imbalance between oxygen supply and consumption (Dachs & Tozer, 2000). This fundamental characteristic of tumor cells is of major clinical importance since hypoxia can predict both tumor progression and poor treatment outcome (Dachs & Tozer, 2000; Vaupel et al., 2001).
Diffusion-limited hypoxia is generally believed to arise from the increasing metabolic demands of the growing mass of cells at increasing intercapillary distances. However, there is now clear evidence that this might not be the only determinant of chronic hypoxia. Another cause is of hypoxia relates to the level of oxygenation of the incoming blood. Indeed, before entering a tumor, a continuous diffusion of oxygen between the blood and the interstitium along the vascular tree accounts for an estimated two-thirds hemoglobin (Hb) deoxygenation (Pittman, 1995). Then, in order for blood to reach the tumor periphery, it must first pass through moderately hypoxic tissues where most of the remaining oxygen in the blood is extracted (Dewhirst et al., 1999).
As a result of this steep vascular gradient of hemoglobin desaturation, many vessels in tumors carry severely deoxygenated blood, so their ability to supply oxygen to the tumor is limited. Additional regions of severe hypoxia in solid tumors result from the uneven partition of erythrocytes in the tumor microvasculature, leading to measurable changes in vascular and perivascular pO2 (fluctuant hypoxia; Dewhirst et al., 1996; Kimura et al., 1996; Hill et al., 1996). Low-frequency red cell flux and pO2 fluctuations in tumors are of high magnitude, which results in a decrease in average tissue oxygenation and a greater variability in local tissue pO2 (Braun et al., 1999; Kimura et al., 1996; Tsai & Intaglietta, 1993). Fluctuating arteriolar diameter might also contribute to flow instability in tumor microvessels (Dewhirst et al., 1996). This, along with the tortuous tumor vasculature accounting for unstable vascular pressures, results in very unstable blood flow, unstable and heterogeneous oxygenation, and areas of fluctuant hypoxia in tumors. Local tumor pO2 can often transiently drop below 3-10 mm Hg (Kimura et al., 1996; Braun et al., 1999), which is considered to be the critical pO2 for radiosensitization (radiobiological hypoxia).
Thus, radiological treatment of tumors is often met with limited success due at least in part to a sub-optimal concentration of oxygen in the tumors. In biological systems, irradiation induces water radiolysis and the subsequent production of the highly reactive oxygen species (OH., O2.−, and H2O2; Mundt et al., 2000). Their most important reactions with biological structures, in terms of therapeutic effect, are those involving DNA, because they are more likely to impair cell survival. They lead to the reversible formation of DNA radicals, which can lead to strand breaks. However, if oxygen is present, then it can react with DNA to produce DNA-O2., which then undergoes further reaction to ultimately yield DNA-OOH (Horsman & Overgaard, 2002). Oxygen-dependent fixation of DNA damage is known as the ‘oxygen effect’. It accounts for the high radiosensitivity of oxygenated areas in tumors; by contrast, hypoxic areas are less sensitive. Upon base damage, DNA reorganization can result in intra- or inter-strand crosslinking, crosslinks between DNA and chromosomal proteins, and single or double DNA strand breaks (McMillan & Steel, 2002). As a consequence, radiation-induced damage is primarily manifested by the loss of cellular reproductive integrity.
Many chemotherapeutic agents are also dependent on cellular oxygenation for maximal efficacy. Cytotoxic alkylating agents, such as the nitrogen mustard alkylating agent melphalan, comprise a class of chemotherapeutic drugs that act by transferring alkyl groups to DNA during cell division. Following this, the DNA strand breaks or cross-linking of the two strands occurs, preventing subsequent DNA synthesis. In a study by Teicher et al., tumor cells in normoxic conditions were more sensitive to melphalan, in contrast to their hypoxic counterparts (Teicher et al., 1985). Under hypoxic conditions, alkylating agents might have less efficacy due to the increased production of nucleophilic substances such as glutathione that can compete with the target DNA for alkylation (Hamilton et al., 2002).
Other examples of drugs directly effected by a lack of O2 include the antibiotic bleomycin and the podophyllotoxin derivative etoposide. Bleomycin does not have maximum efficacy due to the reduced generation of free radicals under hypoxic conditions (Teicher et al., 1981). Etoposide efficacy is reduced due to free radical scavengers, dehydrogenase inhibitors, and dehydrogenase substrates, which prevent the formation of single-strand breaks, thereby decreasing the cytotoxic effects of etoposide (Kagan et al., 2001).
Anticancer drugs such as alkylating agents and antimetabolites act mainly during DNA synthesis by causing damage to the DNA and initiating apoptosis. These drugs can therefore have reduced efficacy on slowly cycling tumor cells under hypoxic conditions. DNA-damaging chemotherapeutic agents such as alkylating agents and platinum compounds might also have compromised function due to increased activity of DNA repair enzymes under hypoxic conditions. Hypoxia also increases the production of various proteins that appear to be responsible for drug resistance (Goldstein, 1996; Zhong et al., 1999; Kinoshita et al., 2001; Comerford et al., 2002).
What are needed, then, are new methods and compositions that can be employed for reducing and/or eliminating hypoxic regions of tumors and other cells that are generally treated with therapies that require the presence of oxygen to be maximally effective. The presently disclosed subject matter addresses this and other needs in the art.